For blood treatment the blood of a patient can be guided via an extracorporeal blood circulation, for example in the course of hemodialysis, hemofiltration or hemodiafiltration. In order to obtain access to the patient's vascular system arteriovenous fistulae, shunts or else vascular implantations can be used. The connection of the extracorporeal blood circulation to the patient is usually made via catheters or cannulas or needles, e.g. dialysis cannulas or needles, by which a fistula or a shunt or, respectively, a vascular implantation, for example, is punctured and a fluid communication is established in this way.
At the beginning of or else during blood treatment the case may occur that the venous access to the blood circulation is disturbed when, for example, the needle or cannula gets out of place and the extracorporeal circulation is no longer connected properly, or is no longer connected at all, to the intracorporeal circulation, i.e. the patient's circulation. This may cause problems especially in the case of disconnection of the venous access from the patient's circulation. Unless such disconnection of the venous access is detected in due time, blood continues being withdrawn from the patient via the arterial access but is no longer properly returned to the patient's body after the extracorporeal blood treatment. In the case of common blood flow rates of 300 to 400 ml/min, for example, a critical situation will develop within few minutes.